Among cellulose films, a cellulose acetate film has higher optical isotropy (a lower retardation value) than other polymer films. Therefore, it is common to use a cellulose acetate film in applications requiring optical isotropy, e.g. for a polarizing plate. On the other hand, it is demanded that optical compensation sheets (retardation films (or phase-contrast films or phase difference films)) for use, for example, in a liquid crystal display device, have optical anisotropy (high retardation value), on the contrary. Therefore, a synthetic polymer film, e.g. a polycarbonate film or polysulfone film, which has a high retardation value, is commonly used as the optical compensation sheet.
As described above, in the technical fields of optical materials, a synthetic polymer film is used when it is required for a polymer film to have optical anisotropy (high retardation value), and a cellulose acetate film is used when it is required for a polymer film to have optical isotropy (low retardation value), in general.
However, in these days, another-type of cellulose acetate film having a high retardation value is demanded, so that it can also be used in applications for which optical anisotropy is required, and technologies to meet this are proposed (for example, European Patent Application Publication (Laid-Open) No. EP0911656A2). In the above EP0911656A2, an aromatic compound having at least two aromatic rings, and particularly a compound having a 1,3,5-triazine ring, is added, and stretching is carried out, in order for a cellulose triacetate film to have a high retardation value, in practice.
Generally, it is known that a cellulose triacetate is a high-molecular raw material that can be stretched (or orientated) with difficulty, and it is difficult to make the cellulose triacetate have a large birefringence. However, in the above EP0911656A2, additives are orientated at the same time by stretching, thereby making it possible to increase birefringence and attaining a high retardation value. This film has such a merit that an inexpensive and thin-film liquid crystal display device can be obtained, because this film can double as a protective film of a polarizing plate.
In recent years, making thinner films for liquid crystal cells has been essential to creating lighter-weight liquid crystal displays, reducing manufacturing cost of liquid crystal displays, or increasing response speed of liquid crystal displays. The optical performance required for an optical compensation sheet is, therefore, a higher Re retardation value, and it has become necessary to provide a film having a lower Rth retardation value.
However, the present inventors, having made earnest studies as to the method disclosed in EP0911656A2, have, as a result, clarified that this method has the problem that it is difficult to set the Re retardation value and Rth retardation value individually, they cannot be set compatibly by the aforementioned method. There are, besides the above EP0911656A2, patent documents, for example JP-A-2001-116926 (“JP-A” means unexamined published Japanese patent application), which disclose techniques concerning the optical performance of a phase-contrast film. In the publications, no method of making a desired Re value and a desired Rth value compatible with each other is clearly described.
In the case of considering that the thickness of a film is fixed, the retardation of the film is determined by the refractive index and amount of a raw material and the orientation state. In the method disclosed in the above EP0911656A2, the retardation of the film is determined by the three-axis refractive indexes and orientation state of the cellulose triacetate, as well as the refractive index, amount to be added, and orientation state of the disk-like compound serving as an additive.
The Re retardation value and the Rth retardation value are both defined as refractive indexes in three-axis directions, and therefore the ratio Re/Rth is mostly determined by an additive that largely contributes to the development of retardation. By investigating the Re/Rth ratio vs. the stretch ratio (or orientation ratio), it was found that both are proportional to each other, and the Re/Rth ratio is increased as the stretch ratio is increased. In the case of varying the amount to be added, the same proportional relation is also obtained: The Re/Rth ratio is increased as the amount to be added is increased. The gradient of the Re/Rth ratio to the stretch ratio is determined by the raw material to be added. It was found that, in the case of the disk-like compound described specifically in the above EP0911656A2, the gradient is small.
The orientation states of cellulose triacetate and additives, which orientation determines the retardation value, differ depending on the stretching method. Generally, a roll stretching method and a tenter stretching method are known, as examples of a monoaxially stretching method. In the former method, the width of a film is shrunk, so that Re tends to be developed. In the latter method, a film is stretched in the transverse direction under the condition that the conveying direction is limited, and Re is scarcely made large. As a result, the Re/Rth ratio to the stretch ratio is smaller in the latter case than in the former case. Because the tenter stretching method has a tendency to decrease the dispersions of film thickness and optical performances within a given plane, it is suitable as a method of producing an optical compensation sheet for a liquid crystal display device. When this method is applied to the aforementioned disclosed example, the increment of the Re/Rth ratio is about 0.01 or less, per 1% of the stretch ratio. When the target value of Re is close to the target value of Rth and the Re/Rth ratio is about 0.5, the stretch ratio must be 50% or more. It is difficult to actually attain this stretch ratio stably in the case of cellulose triacetate film resistant to stretching.
Also, as to the amount to be added, it is difficult to attain the optical performances to be intended by a possible increase in the amount to be added.
Other and further features and advantages of the invention will appear more fully from the following description.